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CLS (Cumulative Layout Shift): definition

SEO

By Alvyn Detavernier, founder · published October 28, 2026

Definition

CLS (Cumulative Layout Shift) measures a page's visual stability by adding up unexpected movements of elements during a visit, like a button that jumps down just as you try to tap it. Google rates CLS as good at 0.1 or less; anything above 0.25 is considered a poor experience.

Also known as: Cumulative Layout Shift, layout shift score

How the score is calculated

Each layout shift scores impact fraction multiplied by distance fraction. The impact fraction is the share of the viewport covered by the unstable elements, before and after they move. The distance fraction is the greatest distance any of them moved, divided by the viewport's largest dimension.

Shifts are grouped into session windows: bursts of shifts less than one second apart, lasting five seconds at most. A page's CLS is the score of the worst window during the visit.

Shifts that do not count

Shifts that happen within 500 milliseconds of a click, tap or key press are treated as expected and excluded, so opening an accordion menu does not hurt the page. Scrolling and pinch-zooming do not count as qualifying input. Animations built with the CSS transform property do not trigger layout shifts at all.

The most common causes

Images and iframes without width and height attributes.
Ad slots or embedded videos with no reserved space.
Cookie banners or promo bars inserted at the top, pushing content down.
Web fonts swapping in with different dimensions than the fallback font.
Content injected above what is already visible, such as reviews loaded late.

How to fix CLS

Always declare image dimensions or a CSS aspect ratio, reserve a minimum height for ad slots and widgets, display banners as fixed overlays rather than inserting them into the page flow, adjust fallback font metrics so they take the same space as the final font, and animate with transform instead of changing an element's position.

To reproduce a problem, throttle the network to a slow mobile profile in Chrome DevTools: the Performance panel highlights each layout shift and the element that moved, which is often not the one you suspected.

Example

Worked example: a block covering 50% of the viewport moves down by 25% of the screen height. The area affected before and after equals 75% of the viewport, so the shift scores 0.75 × 0.25 = 0.1875. That single shift is enough to push the page past the 0.1 threshold.

Sources

Cumulative Layout Shift (CLS), web.dev (Google Chrome)

Frequently asked questions.

Does a cookie banner hurt CLS?

Yes, if it is inserted at the top of the page and pushes content down after rendering. No, if it appears as a fixed overlay on top of existing content. How your consent tool displays its banner is a performance setting as much as a privacy one.

Do I need a CLS score of zero?

No. The target is 0.1 or less for 75% of visits. A zero score is common on simple pages, but a few tiny shifts that nobody notices have no real effect. Focus on visible jumps that make people tap the wrong thing, especially on mobile.

Related terms.

The concepts that go with it, to round out the definition.

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Core Web Vitals

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LCP (Largest Contentful Paint)

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INP (Interaction to Next Paint)

INP (Interaction to Next Paint) measures the delay between a user interaction (a click, a tap or a key press) and the next visual update on screen,...

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